Menino, 69, was diagnosed with dehiscence ptosis, a condition caused by the slow stretching of tiny tendons that hold up the upper eyelids.

"Over time, the tendons can stretch or detach from their anchor point on the eyelids," said Dr. Mami Iwamoto, the Boston-based ophthalmologist who performed the procedure Wednesday. "If that occurs, the lids hang lower than they should."

The upper eyelids usually hang two millimeters above the pupil, the tiny window through which light enters the eye. But in Menino's case, the window was partially blocked.

"It's like there's a shade coming down into that view," said Iwamoto, describing how Menino's eyelid covered half of his pupil. "People often compensate by raising their brows to pull the lids up or by tipping their head back to look under the shade, but he reached a point where he'd be much better off if the lid position was higher."

Menino was awake but sedated for the hour-long surgery, called blepharoplasty, to shorten the stretched-out tendons through tiny incisions in his eyelids. And while the outpatient operation went off without a hitch, swelling and bruising will force the Mayor to lie low for a week.

"He'll feel fine, but it's an awkward situation for making public appearances," said Iwamoto.

Because eyelids can droop with age, some people opt for blepharoplasty for cosmetic reasons. A February 2012 study found that face and eyelid lifts could knock 7.5 years off a person's estimated age. But Menino's motivation was not cosmetic, Iwamoto said.

"It was purely medical need."

Some blepharoplasty patients require additional surgery to tweak changes in tendon length that occur during the healing process.

"I told the Mayor, 'Most of time, people heal exactly the way we planned. But in 10 to 15 percent of cases, we may need to make adjustments,'" Iwamoto said. "He's such a nice gentleman. But he joked that if that happens, 'You're in big trouble!'"

Menino is no stranger to surgery. Since he was elected in 1993, the mayor has gone under the knife at for a cataract, a torn tendon and two tumors, his office confirmed.